High-density polypropylene and polyethylene conduit are often laid underground in long lengths. Splices are required to join one length of conduit to the next. Fiber optic cables, or the like, are then installed in the length of conduit. Relatively smooth interior surfaces are required throughout the conduit length, including the splices, in order to facilitate the installation of the cable. The joints or splices need to be nearly leakage free since air pressure is often used to install (blow) the cable through the conduit. Therefore, the coupling must not obstruct the interior surface of the conduit and must provide a nearly leakage free joint.
U.S. Pat. No. 6,663,145 issued to Lyall et al, U.S. Pat. No. 6,231,090 issued to Fukao et al, and U.S. Pat. No. 5,947,532 issued to Lorenz are all examples of such couplings.
Another example is shown in a brochure from Cabletec®, with a title “Barbed Ring Compression Couplings”, which describes a coupling that joins the ends of two relatively soft plastic conduits. Internal circumferential barbs are provided that grip the inserted conduit and provide separation resistance to axial forces that can be applied to the conduit. Additionally, the barbs provide a seal to prevent air escape during cable installation and to prevent contaminants from entering the conduit. Central to the inside diameter of the coupling is an internal flange that limits the distance of insertion of the conduit from either end of the coupling. The coupling is made of aluminum alloy. Generally, an appropriate tool or mechanism is used to force the conduit into the coupling during the installation process.
A significant element that is missing in these referenced couplings is the lack of ability to know when the conduit is properly engaged into the coupling.